(1) Field of the Invention
The present invention relates to a process for forming a ceramic thin film on a ceramic body. More particularly, the present invention relates to the process for forming a ceramic thin film to be suitably used as a solid electrolyte film, an interconnector film or the like of a cylindrical solid oxide fuel cell.
(2) Related Art Statement
Recently, fuel cells have attracted public attention as power generators. This is because the fuel cell is a device capable of directly converting chemical energy possessed by a fuel to electric energy, and is free from the limitations of the Carnot cycle. Therefore, the fuel cell has an essentially high energy conversion efficiency, causes less pollution, can use a variety of fuels (naphtha, natural gas, methanol, coal-reformed gas, etc.), and the power-generating efficiency is not influenced by the scale of the power-generating plant. Thus, the fuel cell is an extremely promising technique.
In particular, since the solid oxide fuel cells (hereinafter referred to briefly as "SOFC"s) operate at high temperatures around 1,000.degree. C., their electrode reaction is extremely active. Thus, a catalyst of noble metal such as precious platinum is not required at all. Further, since polarization is small, and output voltage is relatively high, energy converting efficiency is far greater as compared with those in other fuel cells. Furthermore, since the SOFC is entirely constituted by solid structural materials, the SOFC has long stability and long use life.
In the above-mentioned SOFC, it is necessary that a solid electrolyte thin film and/or an inter-connector thin film is provided on a cylindrical porous electrode-supporting body. The term "porous electrode-supporting body" may include a porous air electrode itself, a porous fuel electrode itself, and a porous supporting body on which a porous air or fuel electrode is formed. The porous electrode-supporting body may be solid or hollow. Heretofore, thin film-forming processes such as a chemical vapor deposition process (CVD process) and an electrochemical vapor deposition process (EVD process) have been known to form such solid electrolyte thin films. However, these conventional processes have problems in that thin film-forming devices become bulky, and the treatable area and treating speed are too small. For this reason, these processes have defects in that cost is high, and the area of the solid electrolyte film is difficult to increase. In the case of the EVD process, the substrate is limited to the cylindrical shape only.
Since plasma spraying advantageously enables simple formation of thin and relatively dense films at high film-forming speeds the plasma spraying process has been conventionally used for the production of the SOFCs (Sunshine 1981, Vol. 2, No. 1). However, since the plasma-sprayed film has generally a problem that the gas-tightness is poor, if a solid electrolyte film of the above-mentioned SOFC is formed by plasma spraying, the gas-tightness of the film is insufficient. Consequently, a fuel leakage occurs, that is, hydrogen, carbon monoxide or the like leaks through the solid electrolyte film, during the operation of the SOFC. Thus, the electromotive force per cell unit of the SOFC becomes smaller than, for example, lV in ordinary cases, so that output drops and convention rate of the fuel to the electric power decreases.
In addition, a technique is known, which forms an interconnector thin film by using the EVD process. However, as is the same with the formation of the solid electrolyte thin film, the film-forming device becomes bulky, and the treating area and the treating speed are too small, with the result that the cost rises, and the mass productivity becomes difficult.